Mobile ad hoc network (MANETs) is an emerging area with practical applications. One such field concerns mobile ad hoc networks (MANETs) in which mobile nodes organize themselves in a network without the help of any predefined infrastructure. Securing MANETs is an important part of deploying and utilizing them, since them are often used in critical applications where data and communications integrity in important. Existing solutions for wireless networks can be used to obtain a certain level of such security. Nevertheless, these solutions may not always be sufficient, as ad-hoc networks have their own vulnerabilities that cannot be addressed by these solutions. To obtain an acceptable level of security in such a context, traditional security solutions should be coupled with an intrusion detection mechanism. We propose using a quantitative method to detect intrusion in MANETS with mobile nodes. Our method is a behavioral anomaly based system, which makes it dynamic, scalable, configurable and robust. Finally, we verify our method by running ns2 simulations with mobile nodes using Ad-hoc ondemand Distance Vector (AODV) routing. It is observed that the malicious node detection rate is very good, and the false positive detection rate is low.